This invention relates to alumina containing refractories having a volatilized silica binder. It has been known that a volatilized silica binder produces superior characteristics when used in alumina refractory mixtures. The properties which may be improved by the inclusion of the proper proportions of volatilized silica include modulus of rupture, apparent porosity, linear change, volume change and spalling.
Volatilized silica, which is also referred to as fumed silica, is an amorphous silica which has been deposited from a vapor phase. A typical silica of this type results from the reduction of silica to form silicon alloys such as ferrosilicon. A similar silica fume can also be produced by reducing quartz with carbon or other suitable reducing agent, treating the vaporous products of the reduction with an oxygen yielding gas, and condensing the silica in finely divided form. An analysis of such a silica would run at least 90 percent SiO.sub.2 and normally runs about 95 percent, with between 2 and 3 percent of total FeO, MgO and Al.sub.2 O.sub.3.
Refractories of this type are prepared by grinding, screening and blending the desired alumina material to give the desired alumina content with the remainder consisting essentially of the other natural constituents of the alumina ore used. The exact screen sizing to be used is dependent upon such factors as the raw materials used and the purpose to which the refractory is to be put but this is a matter well within the knowledge and skill of those familiar with the refractory field. It is then conventional practice to blend into the batch the volatilized silica in an amount from at least about 1 percent up to about 10 percent by weight of the batch. This volatilized silica is added in the dry form. A temporary binder, of which a wide variety are known and used in the manufacture of refractory materials, is then added to the batch and the batch is tempered with water to give the desired consistency. For a further discussion of the use of volatilized silica in alumina refractories, reference may be had to U.S. Pat. Nos. 3,067,050 and 3,652,307. Refractories of this type may be employed to form brick or they may be used as ramming or gunning materials.